Field of the Invention
The present invention relates to a process for producing continuously cast steel slabs and blooms free from surface defects and requiring substantially no surface conditioning.
In continuous casting, it is very important to reduce the friction between the mold wall and the solidified shell of the strand, so as to prevent the shell from sticking to the mold wall, and thereby prevent "break out." For these purposes, the so-called oscillation mold which oscillates up and down has been used to reduce the friction between the mold wall and the strand shell.
In conventional oscillation mold casting processes, an oscillating mold which oscillates in sine-curved strokes and which is of simplest mechanical structure, as disclosed in "Tekko Binran II" (Handbook of Iron and Steel), third edition, page 638, published by Japan Iron and Steel Association has been most widely used, and the oscillation is such that the maximum speed of the downward motion of the mold becomes higher than a given withdrawal speed of the strand. Thus as shown in FIG. 2, the withdrawal speed (mm/min.) of the strand is maintained constant, while the oscillation rate W(mm/min.) of the mold is W=.pi..multidot.S.multidot.f sin (2.pi..multidot.f.multidot.t) in which S represents the oscillation stroke (mm), and f represents the oscillation cycle (c/min.), and t represents the time (min.). The oscillation is in a sine curve, and the maximum speed of the downward movement .pi..multidot.S.multidot.f is larger than the strand withdrawal speed V.
Supposing the time during which the mold moves downward is "t.sub.p," and the time (healing time) during which the downward movement speed of the mold is larger than the withdrawal speed of the strand is "t.sub.h," it is usually designed that the ratio of "t.sub.h " to "t.sub.p " (the ratio is usually called "negative strip") is maintained in the range of from 60 to 80%.
Most commonly adapted oscillation conditions are: oscillation cycle: 60-90 c/min.; oscillation stroke: 6-10 mm.
In conventional continuous casting using a sine-curve oscillation mold, it has been considered to be a key point, for the prevention of break outs, to maintain the healing time in a certain range so that friction between the mold wall and strand shell is reduced. For maintaining the healing time in a certain range, the three factors, the negative strip, the oscillation cycle, and the oscillation stroke must be adjusted other than the strand withdrawal speed which is maintained constant during the casting operation. In this connection, a higher oscillation cycle has been conventionally considered to be advantageous for consistent supply of powdered additives in between the mold wall and the strand shell. However, an excessively high oscillation cycle, a negative strip as high as 100% is required. Therefore, in the conventional art, 60-90 C/min. of oscillation cycle has been commonly used, and the other two factors, the negative strip and the oscillation stroke have been decided as hereinbefore with the oscillation cycle being maintained in the range of from 60 to 90 C/min.
However, it has been revealed that when continuous casting is done under the above conditions, shallow horizontal depression marks, widely known as "oscillation marks" are formed on the strand shell corresponding to each mold oscillation cycle. The oscillation marks are inevitably formed when an oscillation mold is used, and surface defects, such as abnormal structure due to segregation of the nickel content, fine cracks and entrappment of powdered mold additives, are very often caused along the depressed portion of the oscillation marks. These surface defects will be called hereinbelow "oscillation defects."
The mechanism of the occurrence of oscillation defects may be explained as below by reference to FIGS. 1 (a), (b) and (c).
In continuous casting with use of an oscillating mold, it is commonly practised to add powdered additives (herein called "powder") in the mold so as to provide lubricity between the mold wall and the strand shell, and the powder added within the mold is cooled on the strand shell and sticks thereto to form "slag bear." This slag bear tends to depress and deform the meniscus portion of the shell when the downward movement speed of the mold gets larger than the withdrawal speed of the strand during the downward movement of the mold, and when the mold turns to move upward and the meniscus portion of the shell departs from the slag bear, the molten steel flows onto the upper surface of the meniscus portion of the shell and solidifies there with spacing between the mold wall, resulting in formation of oscillation marks. The fine cracks which occur in the depressed portions oscillation marks are considered to be caused when the meniscus portion of the shell is deformed by the slag bear, while the abnormal structure enriched in segregated nickel, and the entrappment of the powder are considered to be caused by the molten steel and the powder flowing onto the upper portion of the meniscus which is deformed when the mold moves upward.
The oscillation defects in the portions of the resultant steel slabs corresponding to the depressed portions of the oscillation marks are seen mostly within the 2 mm depth of the surface of the steel slabs, and these defects appear as pickled surface irregularities and slivers when, for example, stainless steel slabs are directly rolled without surface conditionings, thus considerably degrading the surface quality of resultant steel sheet products. Therefore, conventionally these oscillation defects are removed by grinding at the intermediate step, but the required surface conditionings result in considerable additional production cost and lowered production yield, etc.
It has been further revealed through afterward experiments by the present inventors that additional defects occur when steel slabs free from the oscillation defects are rolled directly without surface conditionings, and it is impossible to assure complete freedom from surface conditionings. Thus, new additional surface defects, such as entrappments surface roughening and depressions, which occur irrespective to the oscillation marks, have been revealed. These defects are old ones which were confronted within the conventional processes, but raised no problem because they were removed during the whole surface grinding required for removing the oscillation marks.
Therefore, even when whole surface grinding is not necessary by eliminating oscillation defects, partial grinding is necessary for removing the additional surface defects in the case where additional surface defects exist.
The present inventors have discovered that these additional defects are caused by the powdered additives.